1. Field of the Invention
The present invention relates to a power converter, and more particularly, to a primary-side controlled power converter with an RC network.
2. Description of Related Art
Pulse Width Modulation (PWM) technique is widely used in power converters to regulate output voltage to an appropriate level. In order to reduce the number of elements in the power converter and dispense with a secondary-side feedback circuit, a primary-side control technique has been developed. To achieve the primary-side control, a reflected voltage on an auxiliary winding of a transformer can be sensed for voltage feedback control. However, the reflected voltage sensed on the auxiliary winding is not as accurate as a signal sensed directly on the secondary-side of the power converter. Moreover, high-frequency noise and ringing exist in the reflected voltage.
FIG. 1 shows a circuit diagram of a conventional primary-side controlled power converter. The primary-side controlled power converter comprises a transformer 10 including a primary winding NP, a secondary winding NS, and an auxiliary winding NA. An anode of a rectifier 40 is coupled to the auxiliary winding NA. One terminal of a storage capacitor 45 is connected to a cathode of the rectifier 40. The other terminal of the storage capacitor 45 is connected to ground. A switching controller 100 is coupled to sample a reflected voltage VA of the auxiliary winding NA of the transformer 10 through a low-pass filter. The low-pass filter includes resistors RA, RB and a capacitor C2. The resistor RA is connected to the auxiliary winding NA to receive the reflected voltage VA. The capacitor C2 is connected to a join of the resistors RA and RB. The capacitor C2 is used for filtering noises and providing a path with low impedance for EMI. A reflected signal VS is generated at a join of the capacitor C2, the resistor RA, and the resistor RB and received by the switching controller 100. The switching controller 100 generates the switching signal SW based on the reflected signal VS to switch the transformer 10 by switching a power transistor 20. When the power transistor 20 is turned off and the switching signal SW is during an off-time period, the reflected voltage VA is generated accordingly. The waveforms of the switching signal SW and the reflected signal VS are shown in FIG. 2. The transfer function of the reflected signal VS and the reflected voltage VA can be expressed as follows:
                                                                                          V                  S                                                  V                  A                                            =                                                Z                  2                                                                      R                    A                                    +                                      Z                    2                                                                                                                          =                                                                    R                    B                                                        1                    +                                                                  SR                        B                                            ⁢                                              C                        A                                                                                                                                  R                    A                                    +                                                            R                      B                                                              1                      +                                                                        SR                          B                                                ⁢                                                  C                          A                                                                                                                                                                                            =                                                R                  B                                                                      R                    A                                    +                                      R                    B                                    +                                                            SR                      A                                        ⁢                                          R                      B                                        ⁢                                          C                      A                                                                                                                                              =                                                                    R                    B                                                                              R                      A                                        +                                          R                      B                                                                      ×                                  1                                      1                    +                                          S                      ⁢                                                                                                    R                            A                                                    ×                                                      R                            B                                                                                                                                R                            A                                                    +                                                      R                            B                                                                                              ⁢                                              C                        A                                                                                                                                                    (        1        )                                                      V            S                                V            A                          =                                            R              B                                                      R                A                            +                              R                B                                              ×                      1                          1              +                              j                ⁡                                  (                                      f                                          f                      A                                                        )                                                                                        (        2        )                                          f          A                =                  1                      2            ⁢            π            ×                          (                                                                    R                    A                                    ×                                      R                    B                                                                                        R                    A                                    +                                      R                    B                                                              )                        ×                          C              A                                                          (        3        )            
where the frequency fA is a pole of the transfer function of the reflected signal VS and the reflected voltage VA. When the switching signal SW changes from high level to low level and the power transistor 20 is turned off, the reflected signal VS starts to rise. A settling time TD of the reflected signal VS is determined by the frequency fA.
FIG. 2 illustrates waveforms of the switching signal SW and the reflected signal VS in the conventional primary-side controlled power converter of FIG. 1. After the reflected signal VS has been filtered by the low-pass filter, the rising time and the settling time TD thereof are prolonged. The low-pass filter further limits the maximum frequency of the switching signal SW. Therefore, to reduce the settling time TD and to allow increase of the maximum frequency of the switching signal SW are desired.